Aromatic polyamide TFC RO membranes are ubiquitous in our daily lives finding application in many industrial areas such as desalting of brine, ultra-pure water production, environmental pollution treatment, and the like. The trend for the next generation of such membranes is to require more sophisticated and specified functions of the polymeric materials from which they are constructed to provide for an overall enhanced performance of the membrane. This, in turn, drives the need for so-called “tailor fit” materials whose functions and properties are precisely tuned for the intended application of the membrane.
Tailor fit materials for RO TFC membranes are available through either (i) design and synthesis of totally new polymers forming the thin film discriminating layer of the RO membranes, or (ii) the physical and/or chemical modification of the thin-film. The former approach has produced TFC RO membranes of enhanced water flux but with an accompanying considerable loss of salt rejection, or vice versa. The latter approach results from one of two routes that involve either (i) the post-treatment of the thin-film surface of the membrane with various chemicals, or (ii) the use of additives during the formation of the thin film.
Regarding post-treatment, a number of RO membranes have been coated with either with polyvinyl alcohol (PVA) or a vinyl acetate homopolymer with self-crosslinking functionality (e.g., Vinac™ available from Air Products Polymers, L.P.). Regarding the use of additives, a number of membranes, particularly nanofiltration membranes, have been prepared with polymer additives that presumably have been incorporated in the membrane. Important improvements to the membrane resulting from modification of the exterior surface of the discriminating layer include stabilizing the discriminating layer during long-term operations, and balancing the improvement of rejection against the loss of flow due to the alteration of the membrane transport characteristics.
FIG. 1 is a schematic representation of a cross-section of a commercially successful RO TFC membrane, e.g., an FT-30 TFC RO membrane by FilmTec Corporation of Edina, Minn. The first or top layer is an ultra-thin barrier or discriminating layer typically comprising a crosslinked polyamide of 10-100 nanometers (nm) in thickness. One method of preparing this layer is by the interfacial polymerization of m-phenylenediamine (MPD) in the aqueous phase and trimesoyl trichloride (TMC) in the organic phase.
The second or middle layer typically comprises an engineering plastic, such as polysulfone, and it typically has a thickness of about 40 microns (μm). This second layer provides a hard, smooth (relative to the third layer) surface for the top layer, and it enables the top layer to perform under high operating pressure, e.g., 10 to 2,000 psi.
The third or bottom layer is typically nonwoven polyester, e.g., a polyethylene terephthalate (PET) web, with a thickness typically of about 120 μm. This third or bottom layer is typically too porous and irregular to provide a proper, direct support for the top layer, and thus the need for the second or middle layer.
The RO TFC membrane is usually employed in one or two different configurations, i.e., flat panel or spiral wound. The flat panel configuration is simply the membrane, or more typically a plurality of membranes separated from one another by a porous spacer sheet, stacked upon one another and disposed as a panel between a feed solution and a permeate discharge. The spiral wound configuration is shown schematically in FIG. 2, and it is simply a membrane/spacer stack coiled about a central feed tube. Both configurations are well known in the art.
From the viewpoint of performance efficiency, TFC membranes are usually required to have dramatically enhanced water permeability without sacrificing salt separability. Such aromatic polyamide TFC membranes with excellent water flux and reasonable salt rejection characteristics are formed by the interfacial reaction of MPD/TMC that have been kinetically altered with organo-metals and non-metals to form complexes of the TMC as taught in U.S. Pat. No. 6,337,018. This (i) reduces the rate of the reaction of the TMC by reducing the diffusion coefficient and use steric hindrance to block MPD from the acid chloride sites, and (ii) complexes the TMC to block water from hydrolyzing the acid chlorides.
Unlike the chemically analogous FT-30 membrane, the kinetically modified version based on MPD/TMC interfacial polymerization results in modification in surface morphology and variation in the polymer chain organization during formation of the thin film. The combined effect is to increase the rejection of the membrane and to allow the use of other process variables to influence the rate of reaction and therefore the membrane performance. This approach allows for an increase to the membrane flux by over 100% in certain products, e.g., FilmTec's XLE membranes, due to the reduced residual acid chloride after interfacial polymerization and improved swelling ability of the resulting thin film, and it gives the possibility of compensating for flow loss in future post-treatment of thin-film surfaces.
Many applications using membrane processes could benefit from the availability of a wide range of polymer chemistries, e.g., they could exhibit better performance, more robustness and less fouling, and they could use less expense polymers. However, due to the uncertainty of new chemistry and the reluctance of companies to invest in the development of new polymers, alternate approaches such as the surface modification of widely used polymers have increased in importance.
One of the goals of research and industry in the RO membrane field is to enhance, or at least maintain, water flux without sacrificing salt rejection over a long period of time in order to increase the efficiency and reduce the cost of the operation. Nevertheless, the main difficulty in accomplishing this goal is fouling that produces a serious flux decline over the operational time of the membrane.
The principal types of fouling are crystalline fouling (mineral scaling, or deposit of minerals due to an excess in the solution product), organic fouling (deposition of dissolved humic acid, oil, grease, etc.), particle and colloid fouling (deposition of clay, silt, particulate humic substances, debris and silica), and microbial fouling (biofouling, adhesion and accumulation of microorganisms, and the formation of biofilms). Various approaches to reducing fouling have been used, and these usually involve pretreatment of the feed solution, modification of the membrane surface properties (e.g., the attachment of hydrophobic or hydrophilic, and/or electronegative or electropositive groups), optimization of module arrangement and process conditions, and periodic cleaning. However, these methods vary widely in applicability and efficiency and this, in turn, has required continuous, on-going efforts to solve these problems.
For polyamide RO TFC membranes, fouling from the formation of biofilm on the surface caused by microorganisms has been regarded as of the uppermost importance. Microorganisms, such as bacteria and viruses, in the water to be filtered, as well as other microscopic material, e.g., protein, adhere to membrane surfaces and grow at the expense of nutrients accumulated from the water phase. The attached microorganisms excrete an extra-cellular polymeric substance (EPS), and this, in combination with the microorganism and protein, form a biofilm. Biofilm formation is believed related to the depletion of residual disinfectant concentration, and that biofilm is not formed from disinfectant-treated water, such as chlorinated water containing a residual of 0.04-0.05 milligrams per liter (mg/L) of free chlorine. However, chlorination, although effective for the destruction of microorganisms, generates harmful byproducts such as trihalomethanes and other carcinogens.
Protein, cell and bacterial fouling of the membrane surface occur spontaneously upon exposure of the membrane surface, i.e., the external surface of the discriminating layer, to physiologic fluids and tissues. In many cases, biofouling is an adverse event that can impair the function of RO membranes. Common strategies for inhibiting biofouling include grafting antifouling polymers or self-assembled monolayers onto the membrane surfaces. Many synthetic polymers have been investigated as antifouling coatings, and these have met with variable success in antifouling tests.
One common and prominent example of a material used to render a surface inert to nonspecific protein adsorption in medical devices is poly(ethylene oxide) (PEO), a linear, flexible, hydrophilic and water-soluble polyether. Self-assembled monolayers (SAMS) presenting oligo(ethylene glycol) (OEG) groups (as in HS(CH2)11(EG)nOH)) on a gold surface also prevent the adsorption of proteins, even if the number of ethylene glycol (EG) units present is as low as three. Anti-fouling membranes based on grafted, linear polyalkylene oxide oligomers are known, and they provide an improved resistance to fouling while offering excellent flux and salt passage performance (U.S. Pat. No. 6,280,853).